The present invention relates to a process and apparatus for producing high purity nitrogen by the low temperature rectification of air. More particularly, the present invention relates to such a process and apparatus in which light elements, such as helium, hydrogen and neon, are removed from the high purity nitrogen to produce a nitrogen product of ultra-high purity.
Methods and apparatus for producing high purity nitrogen by the low temperature rectification of air are well known in the art. An example of such a method and apparatus is disclosed in U.S. Pat. No. 4,966,002. In this patent, the high purity nitrogen is produced by a single column low temperature rectification process distinguished by its incorporation of a waste recompression cycle. In such a cycle, two partial waste streams of nitrogen are respectively engine expanded and compressed by a compressor coupled to a turboexpander by an energy dissipative brake. The compressed partial waste stream is introduced into the column to enhance nitrogen recovery and the engine expanded partial waste stream is used within the process as a source of refrigeration. Such process and apparatus produces high purity nitrogen at high pressure and at high thermodynamic efficiencies. The product nitrogen is high purity in that it is lean in oxygen. However, the product does contain light elements such as helium, hydrogen and neon, which, due to their volatility, tend to concentrate in the nitrogen product stream in an amount that represents a ten fold increase as compared with their concentration in the entering air. For most industrial applications of nitrogen, such concentrations of light elements are unimportant. However, in the electronics industry, ultra-high purity nitrogen is required in which the product nitrogen is essentially free of the light elements.
U.S. Pat. No. 4,902,321 discloses a process and apparatus for producing ultra-high purity nitrogen that again is illustrated in connection with a single column apparatus. Within the rectification column, a nitrogen rich vapor is produced at the top of the column while an oxygen rich liquid collects at the bottom of the column. A portion of the nitrogen-rich vapor is passed into a condenser where it is condensed by indirect heat exchange with the oxygen rich liquid. The condensed nitrogen is then returned to the column as reflux. A portion of the nitrogen-rich vapor is passed into a shell and tube heat exchanger. Nitrogen-rich vapor rises in the heat exchanger and is progressively partially condensed to produce a nitrogen rich liquid which also collects at the bottom of the heat exchanger. A stream of the nitrogen-rich liquid is expanded to a lower pressure and is then introduced into the shell side of the heat exchanger. The expansion produces a pressure difference between the entering nitrogen rich vapor and the expanded nitrogen rich liquid to in turn produce heat exchange between the vapor and the liquid. The result of this heat exchange is condensation of the nitrogen rich vapor and vaporization of the expanded nitrogen rich liquid which is removed from the heat exchanger as the ultra-high purity nitrogen product.
As can be appreciated, the addition of a shell and tube heat exchanger adds to plant fabrication costs. As will discussed, the present invention provides a process and apparatus to produce an ultra-high purity nitrogen product that in its most basic form, only minimally increases plant fabrication costs. In fact, the present invention can be incorporated into the apparatus used in effectuating the process disclosed in U.S. Pat. No. 4,966,002 with only slight modification to such apparatus.